<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<style>
    .box{
        width: 500px;
        height: 500px;
        margin: 0 auto;
        border: 1px solid red;
        display: flex;
        flex-wrap: wrap;

    }
    .item{
        width: 200px;
        height: 50px;
        border: 1px solid red;
    }

</style>
<body>
    <div class="box">
        <div class="item"></div>
        <div class="item"></div>
        <div class="item"></div>
        <div class="item"></div>
        <div class="item"></div>
        <div class="item"></div>
    </div>
</body>
</html>